We were not allowed out into Hertford alone until we reached the Upper VI,

Hertford was known to be particularly restrictive, but was not unique. My parents lived in Southwold, Suffolk, in the 1970s. Just outside is a school called St Felix. It was a girls' boarding school then, though maybe now it is mixed. I remember noticing that you only saw the girls from the school in town on Saturday afternoons, and never alone. (Possibly 6th formers were allowed un-uniformed, but then one wouldn't have noticed.)